1. Field of the Invention
This invention relates generally to a method and apparatus for enhancing the effects of a stereophonic audio reproduction system and, more particularly, to a method and apparatus for processing stereo signals to enhance the sound field provided in stereo reproduction.
2. Description of the Background
There are now well known numerous systems that are intended to process stereophonic signals during playback in an effort to improve the stereophonic effects that are available. For example, some systems are intended to improve the stereo separation or to place the apparent source of the sounds at locations other than the actual location of the loudspeaker. One system for stereo processing would apply the left channel signal to a specialized left-placement filter and then apply the right channel signal to a right-placement filter. The left input signal and the output of the right-placement filter would be added to form the left channel and the right input channel would be added to the output of the left-placement filter to form the right channel. Such a system can provide some improved stereo effects over a conventional stereo playback system.
On the other hand, normal stereo program material has information that is common to both channels. Thus, in an unprocessed stereo playback system using two loudspeakers this common program information would appear in the center of the stereophonic sound field. It is this common information, or information that is substantially the same in both channels, that when processed according to a system such as described above will result in a general lack of information in and at the center of the sound field. This is so because such common audio information is being simultaneously processed in both the left-placement filter and in the right-placement filter. Thus, the sounds are generally diminished relative to that common material and it has been found that due to such cancellation there is a decrease in the low-frequency information in the processed or so-called enhanced stereo output signals.